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A gunshot wound suffered by David Bain's father Robin was "very unusual" for a self-inflicted wound, a pathologist has told a court.

Pathologist Alexander Dempster has been giving evidence in the High Court in Christchurch of his post mortem examinations of the bodies of David's Bain parents and three siblings.

David Bain, 37, is on trial for the murdering the family on the morning of June 20, 1994. His defence team argue that Robin Bain shot dead his family and then turned the rifle on himself.

Dr Dempster told the court today that the post mortem of Robin Bain, 58, was carried out on June 20, 1994.

Asked about his conclusion of his examinations, Dr Dempster said:"My conclusion was that this was unlikely to be a self-inflicted gunshot wound. But I couldn't exclude it completely."

"My main reason for that was the trajectory, the difficulty of accessing the trigger with the rifle held in that position. These would be the main factors."

The most noticeable injury on his body was a gunshot wound on his left temple, about 1.5cm below the level of his eye socket, with blood clotted around it. It measured 4mm in diameter - a "typical .22 calibre entry wound".

The bullet travelled at a 45 degree angle, and on an upwards trajectory through Robin's head.

It would have caused major damage to his brain and caused death rapidly, Dr Dempster said.

"It would have incapacitated Robin Bain, with death almost certainly following shortly afterwards."

Asked about his views on whether it was a self-inflicted, Dr Dempster said: "I formed the view this was a very unusual wound for a self-inflicted wound. I had never seen a self-inflicted wound that was even close to this trajectory. The majority of self-inflicted wounds in the temple more or less pass across the brain from one side to the other at right angles."

Most self-inflicted gunshot wounds tended to be through the forehead from front to back, or through the mouth or from under the chin.

"The wound is not out of court for a self-inflicted wound. I would say it is at the front end of where you would expect to find self-inflicted wounds in the temple."

"It seems to me that using this particular rifle, with the silencer, makes it awkward for this trajectory. There are much easier ways of triggering that firearm other than holding it to your head at an angle of 45 degrees, front to back."

Dr Dempster said that, in a reconstruction with the same rifle pressed against his left temple, it would seem impossible to fire the rifle with his right arm.

However using his left arm, he was able to reach the trigger with the tip of his middle finger and activate it.

His height and armspan would have to be compared with that of Robin Bain.

A smear of blood was found on Robin Bain's forehead that was not there when his body was first seen at the Bain house, but may have been caused in transport to the mortuary, Dr Dempster said.

He had a normal overnight collection of urine in his body that had not been passed, Dr Dempster said. This evidence is considered important as to how long Robin had been out of bed on the morning of the killings, and whether he would have gone to the toilet.